Boron carbide ceramics can be expected to provide products having lightweight and excellent mechanical properties owing to the possession of characteristics that it exhibits extremely high hardness next to diamond and cubic boron nitride and its bulk specific gravity (density) is about two thirds or lower of that of alumina ceramics, one of typical ceramics. Boron carbide ceramics have been used over years, for example, in products such as abrasion resistant materials, e.g., wire drawing dies and blasting nozzles, and parts or components required to have high impact stress resistance. As boron carbide shows a high modulus of elasticity, its specific rigidity (the degree of its deformation per unit density) defined by its modulus of elasticity and bulk specific gravity as parameters is also high compared with not only ceramics but also carbon-based composites, and its superiority is recognized even as members that rotate at high speeds, such as steppers employed in semiconductor manufacturing equipment.
However, boron carbide is significantly inferior in sinterability as its bonds have strong covalent nature. Boron carbide has, therefore, been accompanied by the problem that good boron carbide ceramics cannot be produced economically with ease. More specifically, boron carbide is significantly inferior in sinterability even when compared with silicon carbide which is a representative carbide the sinterability of which is generally considered to be poor, and hence, it has been difficult to heat boron carbide under normal pressure without application of pressure. As a sintering method practiced upon production of high-purity boron carbide ceramics, pressure sintering such as hot pressing or gas-pressure sintering is commonly used. With such a method, however, manufacturing equipment and manufacturing cost become large, thereby failing to economically provide good boron carbide ceramic products. More specifically, the conventional pressure sintering requires pressurization equipment so that the initial cost and running cost become enormous compared with those required for pressureless sintering. Moreover, green bodies which can be heated are limited to those having simple configurations because of the application of pressure. To produce a machine part or the like of a complex configuration, for example, machining with an expensive diamond tool or the like is needed after obtaining a boron carbide ceramic of a simple configuration, and therefore, high working cost is required. It has, accordingly, been difficult to economically provide a boron carbide ceramic product of a complex configuration by conventional normal sintering. As is readily appreciated from the foregoing, the production of a boron carbide ceramic by pressure sintering is accompanied by a problem that diverse restrictions are also imposed on aspects other than equipment, to say nothing of the equipment, and technologies, which have been cultivated in normal sintering widely employed in industry, cannot be applied as they are.
Under such circumstances, it is practiced to provide a boron carbide ceramic with improved sinterability by forming it into a cermet (a composite of a ceramic and a metal). For example, it is disclosed in Patent Document 1 that a high-density and high-strength ceramic having a density of at least 85% of theoretical density can be obtained in accordance with pressureless sintering by blending a boron carbide powder, a silicon carbide powder and aluminum and heating the resultant blend into a cermet. However, the material obtained by the cermet process can hardly be considered to take advantage of the excellent properties inherent to boron carbide ceramics although it can show density and strength to some high extent.
With a view to making an improvement in sinterability, attempts have also been made to achieve densification and to make good use of the mechanical properties of boron carbide to some extent by forming boron carbide and alumina (aluminum oxide) into a composite (see Patent Document 3). However, these attempts both include a problem in that the bulk specific gravity becomes high compared with that of boron carbide alone and the inherent properties of boron carbide ceramics are impaired, because they both require as much as several percents of an additive to promote sintering so that a material higher in bulk specific gravity than boron carbide alone is added.
It was, therefore, desired to develop a normal sintering process capable of easily and economically producing a boron carbide ceramic, and a variety of proposals have been made to date. There are processes that similar to processes of obtaining silicon carbide by normal sintering, permit normal sintering by incorporating a sintering additive in a green body, and proposals have been made to date about the use of various sintering additives. There are, for example, processes that add aluminum, an aluminum alloy or an aluminum compound as a sintering additive in a raw material for a green body (see Patent Document 4), processes that add an aluminum-containing material as a sintering additive and also add an additive such as boron nitride (see Patent Document 5). It has also been proposed to obtain a dense boron carbide ceramic by using a small amount of carbon as a sintering additive and controlling a heating atmosphere with H2/He (see Patent Document 6). It is proposed in Patent Document 5 to conduct sintering in a partial pressure atmosphere, which contains an aluminum-containing material as a sintering additive, by incorporating the sintering additive in a green body and further allowing the sintering additive to coexist together with the green body. In addition, there is also a proposal that obtains a dense boron carbide ceramic by coating surfaces of a boron carbide powder with carbon (see Non-patent Document 1).
Patent Document 1: JP-A-57-156372
Patent Document 2: U.S. Pat. No. 4,195,066
Patent Document 3: JP-A-47-8078
Patent Document 4: JP-A-59-184767
Patent Document 5: JP-A-8-12434
Patent Document 6: WO-A-2006/110720
Non-patent Document 1: J. Mater. Rev., 22(5) 1354-1359 (May 2007)